The present invention relates generally to prosthetic joints and, particularly to a spherical head used in such applications. More particularly, the present invention relates to a method for producing a spherical head having a very smooth surface for prosthetic applications.
The recent advances in the use of implantable prosthetic joints to replace natural joints in humans have produced significant improvements. Many designs for such prosthetic implants, such as hip prosthesis, require a spherical head having a highly polished surface to reduce friction and increase the life of the joint.
It is known to machine parts from bar stock on a ball grinder to produce parts that are spherical, but have a relatively rough surface. The parts are then buffed on a buffer to achieve a polished surface. A problem with the buffing process, however, is that surface irregularities, such as grinding and buffing lines, remain on the finished part.
According to the present invention, a method of lapping a spherical head is provided which reduces surface irregularities on the finished part. The method comprises the steps of rotatably mounting a plurality of lapping fixtures having generally semi-spherical cavities, positioning polishing cloths or liners in the generally semi-spherical cavities, sequentially bringing the spherical head into engagement with the spinning lapping fixtures and varying the angle between the longitudinal axis of the spherical head and the rotational axes of the lapping fixtures between forward and reverse rotations of the spherical head about its longitudinal axis.
In accordance with preferred embodiments, the subject method further includes the step of applying a wetting agent and diamond suspensions varying from a relatively coarse grit to a relatively fine grit to the lapping fixtures. Also in accordance with further preferred embodiments, the subject method includes the step of rinsing the lapping compounds from the spherical head.
In the illustrated embodiments, the subject method further includes the step of positioning a generally daisy-shaped polishing cloth having adhesive backing in each of the semi-spherical cavities. The adhesive backing ensures good engagement between the mating surfaces of the polishing cloths and the respective semi-spherical cavities.
According to the present invention, each lapping fixture comprises a central core having a top end defining a generally semi-spherical cavity and an outer sleeve having an opening adapted for receiving the central core. According to still further preferred embodiments, the subject method further includes the steps of positioning the polishing liners in the respective semi-spherical cavities with the peripheral portions thereof extending over the top ends of the lapping fixtures and mounting the outer sleeves over the central cores to clamp the peripheral portions of the polishing liners in place.
In illustrated embodiments, the lapping fixture includes a plurality of modular inserts for lapping a corresponding plurality of heads having different diameters. Each modular insert has the same predetermined outside diameter to securely fit the opening in the central core of the lapping fixture, but a different diameter semi-spherical cavity.
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the inventions presently perceived.